Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alpha-methylstyrene and vinyltoluene) typically produce products contaminated with various impurities, such as benzene, toluene and the like. These impurities must be removed in order for the monomer product to be suitable for most applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation.
However, it is well known that vinyl aromatic compounds polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions various polymerization inhibitors have been employed.
In general, the compounds which are commercially employed as such polymerization inhibitors are of the dinitrophenolic class. Thus, for example, Drake et al, in U.S. Pat. No. 2,526,567, show the stabilization of nuclear chlorostyrenes employing 2,6-dinitrophenols. Similarly, U.S. Pat. No. 4,105,506, to Watson, discloses the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds.
In addition, it has been disclosed by Butler et al, in U.S. Pat. No. 4,466,905, that, in the presence of oxygen, phenylenediamines in the distillation column together with 2,6-dinitro-p-cresol will reduce the amount of polymerization which occurs.
While dinitrophenols are effective polymerization inhibitors, there are several disadvantages associated with their use, either alone or in blends. For example, dinitrophenols are solids that, if subjected to temperatures above their melting points, are unstable and may explode (see U.S. Pat. No. 4,457,806). Moreover, dinitrophenols are highly toxic, many having a dermal LD.sub.50 (rabbit) in ethylbenzene of less than 2 g/Kg.
While such prior art inhibitors may inhibit the polymerization of vinyl aromatic compounds to some degree, it would be desirable to possess polymerization inhibitors which would more effectively delay the onset of polymerization and/or which would avoid the use of highly toxic compounds such as dinitrophenols.
Recently, it has been disclosed by Kolich, in U.S. Pat. No. 4,633,026, that halogenated vinyl aromatic compounds (such as bromostyrene) may be inhibited from polymerizing by the addition of an amine polymerization inhibitor selected from the group consisting of certain alkyl-substituted phenylenediamine compounds and phenothiazine compounds in the presence of air.
More recently, Abruscato et al in U.S. Pat. No. 4,4774,374 discloses using a vinyl aromatic compound along with an oxygenated phenylenediamine for polymerization inhibition in vinyl aromatic species. In U.S. Pat. No. 4,915,873 Abruscato shows vinyl aromatic compounds stabilized against polymerization using an effective amount of a phenothiazine compound and an aryl-substituted phenylenediamine. Although these inventions do not utilize dinitrophenols, the inhibitors of these inventions require air to function.
Accordingly, it is an object of this invention to provide an improved polymerization inhibitor system for the prevention of polymerization of vinyl aromatic compounds.
It is an additional object of this invention to provide an inhibitor system for the prevention of polymerization of vinyl aromatic compounds, which inhibitor system does not comprise toxic dinitrophenolic compounds.
Another object of this invention is to provide a vinyl aromatic polymerization inhibitor sytem which does not require air to function.
It is a further object of this invention to provide a vinyl aromatic composition which is stabilized against polymerization.
It is yet another object of this invention to provide an improved method for inhibiting the polymerization of vinyl aromatic compounds.